Thermoplastic polymers are important items of commerce, millions of tons being produced annually. Typically they are processed and/or formed into useful shapes by melting the polymer and then allowing it to cool to solidify. This is often called melt processing. One important method of melt processing is extrusion in which the polymer is melted and then pushed through a die having one or more orifices having a particular shape. After exiting the die the molten polymer is cooled and solidified. Among the items which can be made in this fashion are strands (which may be cut into pellets), rods, bars and other (sometimes more complex) profile shapes, fibers, and films.
A common problem in extrusion is the sticking of small amounts of the polymer being extruded to the edges of the die where the polymer exits the die, this edge sometimes being called the die lip (see for instance U.S. Pat. No. 6,164,948). As this polymer builds up the polymer furthest from the die surface is thermally insulated from the (usually) hot die and often solidifies. This buildup is called by numerous names, such as die drips, die drool, die lip buildup, etc., and may be referred to herein by any of these names. This buildup is deleterious because the heated die may cause the polymer to degrade, and/or the polymer buildup typically often contacts and adheres to (thus breaking away from the die surface) the molten polymer stream being extruded and may cause a defect in the extrusion being produced and/or contaminate the extrudate with degraded polymer.
Many methods have been tried to minimize this buildup (see U.S. Pat. No. 6,164,948 and references cited therein), since cleaning of the die face typically causes a shutdown in production, or at least a pause in production, of quality extrusion, usually with a concomitant loss of polymer, which typically becomes scrap. Therefore methods for cleaning of extrusion dies without loss of production are sought.